A wind turbine is a device in which a rotor with aerofoils rotates by wind force thereby to convert wind energy to power of the rotor. The horizontal axis wind turbine is defined as a wind turbine in which the rotation axis of the rotor is located approximately in the horizontal plane. On the ground, a tower is installed to support the rotor at a predetermined height above the ground. At the upper part of the tower, a nacelle is arranged. In the nacelle, the rotation axis of the rotor, a power generator and a controller are accommodated. The power generator generates power upon receiving the rotational force of the rotor.
Between the tower and the nacelle, there is installed a yaw drive unit for turning the nacelle relative to the tower in the horizontal place in accordance with the wind direction in such a manner that the aerofoils are kept against the wind that varies in direction (for example, see PL1). A controller operates the yaw drive unit to control the deviation angle between the wind direction and the rotation axis of the rotor to be a predetermined angle or less. The yaw drive unit has a large ring-shaped gear provided in the tower and a geared motor with pinion provided in the nacelle. The pinion of the geared motor engages with the ring gear of the tower. When the geared motor is turned, the pinion rotates on its axis and rotates around the ring gear. With rotation of the pinion, the nacelle turns in the horizontal plane.